FR3038894A1 - PROCESS FOR PRODUCING CALCIUM ALUMINATES - Google Patents

Abstract

The invention relates to a process for the production of calcium aluminates in an industrial furnace (1), according to which, continuously, is introduced into a tank (15) of refractory material containing a permanently heated melt (11). fine particles of a raw material source of alumina (Al2O3) and / or aluminum (Al) and a raw material source of calcium oxide (CaO) and / or calcium (Ca) having a diameter median d50 less than or equal to 6,000 microns for melting said fine particles of raw material, and is continuously recovered at the outlet of the tank a mass of calcium aluminates (16) liquid.

Description

Technical field to which the invention relates

The present invention relates to the production of calcium aluminates.

It relates more particularly to a continuous manufacturing process for calcium aluminates using an industrial melting furnace.

Technological background

Because of their hydraulic and binder properties, calcium aluminates make it possible to produce cements or concretes with many qualities. Aluminous cements are resistant to aggressive agents and high temperatures. They are at the origin of many technical products such as special mortars, refractory concretes, etc. They can also be used as mineral reagent associated with other components. They are used in various industries, such as the refractory industry, construction chemistry, metallurgical fluxes (trapping impurities from molten metals), or in the pipe and sewerage industry.

The calcium aluminates may have different mineralogical phases such as 3Ca0.Al2O3 (C3A), Ca0.Al2O3 (CA), Ca0.2Al2C> 3 (CA2), Ca0.6Al2C> 3 (CA6) or else 12Ca0.7Al2C> 3 ( C12A7). These mineralogical phases, which account for both the atomic scale structure and the chemical composition of the calcium aluminates, influence the final properties, for example on the reactivity, of said calcium aluminates.

In addition, the final, and especially reactive, properties of calcium aluminate-based products depend in part on the amount of alumina (Al 2 O 3) and / or aluminum (Al) and calcium oxide (or lime). CaO) and / or calcium (Ca) contained in calcium aluminate. Calcium aluminate is often referred to as its mass ratio Al / Ca, ie the ratio of the total mass of aluminum to the total mass of calcium contained in calcium aluminate.

Currently, calcium aluminates are mainly produced by two high temperature processes, namely by sintering or melt process, in cement kilns such as sintering furnaces, rotary flame furnaces, or vertical melting furnaces.

For example, document FR2291162 discloses a process for producing calcium aluminates by sintering, which consists of calcining, that is, heating in the solid state of the calcium source raw materials, for example lime CaO, and aluminum source materials, for example Al 2 O 3 alumina, in a flame rotary kiln at a temperature of between 1400 ° C and 1600 ° C. In general, a rotary kiln consists of a tube, slightly inclined, covered on its inner face with refractory bricks, a flame being disposed at the lower end of the tube. The sources of calcium and aluminum are then introduced into the furnace by the highest end. They are then generally heated at a temperature between 1400 and 1600 degrees Celsius (° C) for a period of about 30 minutes before being discharged in the lower part, near the flame.

Such a sintering process consists of a surface reaction between the powdery raw materials which react together without going through a generalized liquid state.

According to the process described in this document FR2291162, the raw materials used have a particle size of less than 208 micrometers for calcination. The clinker obtained has more than 80% of CA mineral phase.

Also known from FR1013973 is a process for producing calcium aluminates by melting in which the source materials of calcium and aluminum are heated to the liquid state in a flame-spinning furnace at temperatures close to 50.degree. 1430 ° C to 1450 ° C.

Calcium and aluminum source materials used in such a process are calcareous briquettes (CaC03) and ferruginous bauxite - a mineral rock rich in alumina and containing iron, silica and other compounds in varying amounts . The briquettes generally have an average size of between 15 millimeters (mm) and 20 centimeters (cm).

According to the process described in this document FR1012973, the raw material is heated by slowly rotating the oven until a uniform melt is obtained and the melt is recovered immediately after reaching the melting temperatures of the raw materials.

It is also known that a vertical melting furnace can be used to carry out fusion processes.

This vertical melting furnace has a vertical portion whose height can reach ten meters and a generally horizontal portion from which the liquid mass of calcium aluminates obtained is recovered.

More particularly, lime and bauxite blocks are loaded through an opening in an upper zone of the furnace into the vertical portion of the melting furnace and heated by a flame disposed in a lower zone of the furnace. The flame heats the blocks to a temperature close to 1500 ° C to melt and form a liquid mass which is directly recovered by a taphole.

During the process, flue gases form and take a counter-current path to that of the blocks. They are evacuated by a chimney located in the upper zone of the furnace, in its vertical part. These combustion gases, having a temperature greater than 1500 ° C, circulate between the blocks and preheat them.

Prior to being in contact with the flame, the raw material blocks are thus dried, followed by dehydration and decarbonation by the flue gases up the vertical portion of the melting furnace.

Such a process requires the use of raw materials in blocks excluding fine particles that would cause blockages and damage to the vertical portion of the melting furnace.

Thus, the melt process has the disadvantage of using bauxite blocks which are less and less available on the market. In addition, during extraction, the production yield of the bauxite blocks is low. In fact, for every 100 tonnes of ore mined, only 10 tonnes of crude bauxite are obtained, including 8 tonnes of fine particles (which can not be used by the melting process described above), and 2 tonnes of bauxite in useable blocks, together with 90 tons of waste rock that can not be used by industries.

Object of the invention

In order to overcome the aforementioned drawback of the state of the art, the present invention proposes a process for manufacturing calcium aluminates by melting which does not require the use of bauxite blocks and which makes it possible to recover the fine particles. extracted raw materials that are available on the market.

More particularly, the invention provides a process for the production of calcium aluminates in an industrial furnace, according to which: a) a fine particle is introduced continuously into a refractory tank containing a permanently heated melt. a raw material source of alumina (Al2O3) and / or aluminum (Al) and a raw material source of calcium oxide (CaO) and / or calcium (Ca) having a median diameter d50 lower or 6,000 μm to melt said fine particles of raw material, and b) a mass of liquid calcium aluminates is continuously recovered at the outlet of the tank.

The median diameter d 50 of any set of particles is a quantity representative of the statistical distribution of the sizes of these particles, in other words the particle size of this set of particles.

The median diameter d50 is a reference diameter defined as the diameter below which 50% of the fine particles used is present, in mass relative to the total mass of all of said fine particles.

In other words, for a set of fine particles having a median diameter d 50, 50% by weight of these fine particles has a diameter less than this median diameter d 50, and 50% by mass of these fine particles has a diameter greater than median diameter d50 given.

The term "diameter" here means the largest dimension of the particle, whatever its shape.

The median diameter d50 of a set of fine particles is obtained from a granulometric curve representing the statistical distribution of the size of each of the fine particles of this set.

In practice, the median diameter d50 of a set of fine particles can be determined by various techniques, such as the sedimentation method (detection by X-ray absorption) or the laser diffraction method (ISO 13320).

In the context of the present invention, the size of the fine particles is measured according to the ISO 13320 standard by the laser diffraction method with, for example, a Mastersizer type 2000 or 3000 laser type granulometer marketed by the company Malvern.

Advantageously, the manufacturing method according to the invention uses fine particles of raw material, not valued in current technology, from the extraction and processing of ore for melting of calcium aluminates.

Thus, the method according to the invention makes it possible to use unusable raw materials in the current melting process.

In addition, here, these fine particles are directly immersed in a bath of calcium aluminates heated to a temperature allowing them to melt. A homogeneous liquid calcium aluminate mass is thus recovered at the tank outlet, that is to say without being unfused.

"Unmelted" means particles of raw material still in solid form, which would not have reacted during the process.

In addition, according to the process according to the invention, and unlike the existing processes of the prior art, it is possible to avoid carrying out the preliminary stages of dehydration and decarbonation of the raw materials used, this dehydration and decarbonation intervening directly in the melt. Moreover, according to the process according to the invention, the bubbles generated by the decarbonation of the raw materials during their melting in the melt naturally participate in the mixing of this melt. This natural mixing mixes the fine solid particles with the liquid material contained in the melt, thus promoting the melting of said fine particles. Therefore, this natural stirring contributes to the improvement of the homogeneity of the calcium aluminate liquid mass obtained. The process according to the invention thus stabilizes the quality of the finished products obtained.

Advantageously, in step a) of the manufacturing method according to the invention, the melt is placed under a reducing atmosphere comprising carbon monoxide (CO).

In particular, said reducing atmosphere comprises on average about 0.1% to 100% carbon monoxide (CO).

Advantageously, the reducing atmosphere in which the melt is placed makes it possible to control, at least in part, the mineralogical phases of the calcium aluminates obtained, for a given proportion of lime and alumina contained in said calcium aluminate.

"Mineralogical phases" describe both the atomic scale structure and the chemical composition of calcium aluminate. For example, these mineralogical phases are: the C3A phase (3Ca0.AI203), the CA phase (Ca0.AI203), the CA2 phase (Ca0.2Al2O3), the CA6 phase (Ca0.6Al2O3) or the C12A7 phase (12Ca0 .7AI203).

In particular, it turns out that the C12A7 mineralogical phase influences the reactivity of the calcium aluminate used as a hydraulic binder, that is to say as a material capable of reacting with water to form a paste that hardens cold by agglomerating aggregates between them. Hardening of the hydraulic binder is also called "hydraulic plug".

Specifically, the C12A7 mineralogical phase is an accelerator for setting calcium aluminate. In other words, calcium aluminates harden all the faster in contact with water they contain a large proportion of C12A7 mineralogical phase, compared to other mineralogical phases optionally contained in said calcium aluminate.

Calcium aluminates can be used for various applications, according to which a user will prefer a quick or slow hydraulic setting, it is particularly interesting to be able to control the proportion of C12A7 mineralogical phase contained in calcium aluminates. Advantageously, the method according to the invention allows such control. Other nonlimiting and advantageous features of the process according to the invention, taken individually or in any technically possible combination, are as follows: the temperature of the melt of the calcium aluminates is between 1300 ° C. and 1 ° C. 700 ° C; the melt temperature of the calcium aluminates is between 1400 ° C and 1600 ° C; - The residence time of said fine particles of raw material in said melt of calcium aluminates is less than 24 hours; - The residence time of said fine particles of raw material in said melt of calcium aluminates is between 30 minutes and 9 hours; in step a), the source material that is the source of alumina and / or aluminum introduced into the tank is chosen from: bauxite, corundum wheels, catalyst supports, refractory bricks, hydroxides, metallurgical aluminas, calcined and melted aluminas, by-products of the aluminum die and non-conforming high-alumina manufacturing compounds or mixtures thereof, and the raw material source of calcium and / or calcium oxide introduced in the tank is chosen from: limestone, lime and by-products from consuming processes of limestone and lime such as slags or slags of steelmaking or electrometallurgy, or a mixture thereof; the fine particles of raw material have a median diameter d 50 of between 100 μm and 1000 μm; the fine particles of raw material have a median diameter d 50 of between 150 μm and 500 μm; after step b), the mass of liquid calcium aluminates recovered at the tank outlet is cooled; - the cooling takes place naturally; the mass of cooled calcium aluminates is milled to form a calcium aluminate cement; said fine particles of alumina (Al2O3) and / or aluminum (Al) source raw material and source material of calcium oxide (CaO) and / or calcium (Ca) are introduced into the sub-tank; the shape of a loose powder.

Detailed description of an example of realization

The following description with reference to the accompanying drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be achieved.

In the accompanying drawings: FIG. 1 is a granulometric curve of a first set of bauxite particles suitable for the process according to the present invention; FIG. 2 is a bar graph showing the distribution of the diameters of a second set of bauxite particles suitable for the invention; FIG. 3 is a bar graph showing the distribution of the diameters of a third set of bauxite particles suitable for the invention; FIG. 4 is a bar graph showing the distribution of the diameters of a fourth set of bauxite particles suitable for the invention; and FIG. 5 is a diagrammatic cross-sectional view of an oven for implementing the calcium aluminate manufacturing process according to the invention.

In the remainder of the description and unless otherwise specified, the indication of an interval of values "from X to Y" or "between X and Y" is understood to include the values X and Y.

Device

FIG. 5 partially and schematically shows an example of furnace 1 adapted to the implementation of the calcium aluminate manufacturing process according to the present invention.

Overall, this oven 1 comprises a horizontal tank 15 - a kind of pool - covered by a vault 5, and a smoke exhaust opening (not shown).

This tank 15 is adapted to contain a melt 11 obtained and maintained in the liquid state by a heating system 10 of the furnace 1.

The tank 15 is also adapted to receive, through an inlet opening 9, solid raw materials 7 and to discharge, through an outlet opening 12, molten materials, namely a mass of liquid calcium aluminates 16. For this purpose, the oven 1 comprises a charging system 2 of the raw materials 7 connected to the inlet opening 9 of the vessel 15, and a discharge system 3 of the molten materials connected to the outlet opening 12 of the tank 15.

In this furnace 1, there is therefore a real flow of material fed by the raw materials 7 introduced continuously through the inlet opening 9, which are transformed in the melt 11 liquid calcium aluminates, themselves evacuated through the outlet opening 12.

In what follows, "residence time" will be used to denote the time that elapses between the introduction of a particle of raw material into the melt 11 of the tank 15 of the oven 1 and its evacuation through the opening output 12,

More precisely, as shown in FIG. 5, the tank 15 here delimits a globally parallelepipedal volume.

The walls of this vessel 15 comprise in particular a hearth 4 constituting the bottom of the tank 15 and a peripheral wall 14 which rises vertically from the hearth 4.

The hearth 4 here has a rectangular shape so that the peripheral wall 14 has four panels arranged in pairs at right angles.

Of course according to other embodiments of the oven, the tank may have a different shape. In particular, the sole and the peripheral wall could have different shapes and form between them different angles to optimize the flow between the inlet and the outlet of the tank but also the distribution of the raw material in the bath of melting and evacuation of the melt.

One of these four panels of the peripheral wall 14, called the entrance panel, comprises the inlet opening 9 of the tank 15 allowing the charging system 2 of the raw materials 7 to access the tank 15.

The panel opposite the input panel, called exit panel, has the outlet opening 12 of the tank 15, also called taphole, for connecting the tank 15 to the evacuation system 3 of the melts.

The passage of the material flow is thus done in the longitudinal direction of the furnace 1, from the inlet opening 9 to the outlet opening 12 of the vessel 15.

The raw material stays longer or shorter in the tank 15 depending on the size of the tank 15.

The laboratory surface of the tank 15, that is to say the internal surface of the tank 15 intended to be in contact with the melt 11, is between 20 m2 and 200 m2, preferably it is equal to about 100 m2.

The walls of the tank 15 and the vault 5 are lined internally with a refractory material chemically inert with the calcium aluminates.

The refractory material of the tank 15 and / or of the vault 5 is chosen from the agglomerated refractory materials or the electrofusion refractory materials and / or a mixture thereof.

More specifically, the nature of the refractory materials used may be a function of their location in the furnace (vault, tank) and associated stresses. Thus, it is possible to use agglomerated or electro-cast refractory materials for the vault 5 and refractory electro-fused materials for the vessel 15.

These two families of agglomerated and electrocast refractory materials are differentiated primarily by their forming processes: a casting-type casting for electro-cast refractory materials and sintering in the case of agglomerated refractory materials.

Thus, the agglomerated refractory materials are sintered ceramic materials, large or small grains, obtained by unidirectional or isostatic pressing, by vibrocolding or by slip casting. They are characterized by an open porosity of up to 20%, reduced in the case of isostatically pressed materials. In this category, there are several chemical compositions of refractory products. The most common chemical compositions of agglomerated refractory materials are summarized in Table 1 below.

These chemical compositions are given as a percentage by weight. The mass percentage of the compounds (MgO, CrC 3, Al 2 O 3, ZrC 2, SiO 2, CaO, FeC 3) is sometimes given in A / B form, which means that the agglomerated refractory material in question comprises, by mass, relative to the total mass of said agglomerated refractory material, from A% to B% of compound.

Table 1

Refractory electro-cast materials have a lower porosity than agglomerated refractory materials. They also have an organization of the crystalline structure to significantly increase their resistance to corrosion.

There are four families of electro-cast refractory materials: Alumina-Zirconia-Silica (AZS), Alumina-Zirconia-Silica-Chromium (AZSC), Very High Zirconia (THTZ) and High-grade

Alumina (HA).

Table 2 below summarizes the chemical compositions of some electro-cast refractory materials. These chemical compositions are given as a percentage by weight. As in Table 1, the mass percentage of the compounds is sometimes given in the form A '/ B', which means that the electrofired refractory material in question comprises, in mass with respect to the total mass of the electrically refractory refractory material, A ' % to B '% of compound.

Table 2

The tank 15 thus formed is adapted to contain the melt 11. This melt 11 is here is molten calcium aluminate bath.

The heating system 10 is adapted to permanently heat the melt 11 contained in the tank 15.

Here, the heating system 10 equips the inner face of the vault 5.

It is preferably a combustion heating system 10 which includes flame burners such as oil burners or gas burners.

For example, it is an air-fueled heating system in which the oxidant is the oxygen (02) coming from the air.

An oxy-fuel heating system could also be provided in which the oxidant would be oxygen (02) from a source of pure oxygen.

In addition, the oven 1 may optionally comprise a heat regenerative system not shown here.

This regenerative system is made of refractory materials such as those used for the vault 5 or the tank 15.

Advantageously, this heat regenerating system is generally associated with the heating system 10 for recycling the combustion energy. It increases the thermal efficiency of the oven. These are, for example, piles of refractory bricks crossed by numerous channels in which the combustion gases flow alternately and air or pure oxygen used for combustion: the gases give up their energy to the bricks which restore it when the passage of air or pure oxygen.

According to a variant of the oven not shown, it could be provided that the heating system of the melt is electric.

This heating system could for example comprise plunging electrodes or electrodes arranged at the bottom of the tank.

These electrodes could be molybdenum electrodes.

Moreover, from a total point of view, the tank 15 surmounted by the vault 5 forms a partly closed enclosure containing the melt 11.

It is possible to choose the composition of the gases contained in this chamber above the tank 15.

Advantageously, here, the partially closed enclosure is adapted to accommodate a gas mixture containing carbon monoxide (CO) which forms a reducing atmosphere above the tank 15.

More precisely, said reducing atmosphere comprises the gases in contact with the surface of the melt 11 contained in the tank 15.

With the furnace combustion heating system 1 described above, namely the air-fired heating system or the oxy-fuel combustion system, the atmosphere in the enclosure naturally contains carbon monoxide (CO) from combustion.

In particular, it is possible to control the carbon monoxide (CO) content of the gases contained in the chamber by precisely controlling the combustion reaction, and in particular the stoichiometry of the oxidizing reagents (02) and fuels (fuel oil, gas).

Advantageously, it will also be possible to add a supply system of the carbon monoxide enclosure (CO) (not shown).

In the furnace 1 having the combustion heating system 10, the carbon monoxide (CO) content is not homogeneous throughout the enclosure, that is to say that it is not identical in all the points of the enclosure. In general, for the reasons of stoichiometry stated above, it is generally stronger near the flame burners.

We will then speak of "average" content of carbon monoxide (CO). This "average" content of carbon monoxide (CO) is evaluated in the gases evacuated from the chamber through the smoke exhaust opening (not shown) of the furnace 1, It is for example measured by a sensor arranged in a smoke exhaust duct into which said smoke exhaust opening.

In the case of the variant comprising the electric heating system, the oven will necessarily be equipped with a system of artificial addition of carbon monoxide (CO).

The electric furnace may also include means for controlling the carbon monoxide (CO) content of the atmosphere contained in the enclosure.

This means of controlling the carbon monoxide (CO) content makes it possible to precisely regulate the content of carbon monoxide (CO) in the enclosure.

Carbon monoxide (CO) is for example injected pure in the chamber where it mixes with the ambient air. It can also be introduced directly in a mixture with air.

It can also be injected pure so that the chamber only includes carbon monoxide (CO). On the other hand, as shown in FIG. 5, the charging system 2 of the oven 1 is connected to the inlet opening 9 of the tank 15.

This charging system 2 comprises a silo 6 in the form of a funnel for storing or even homogenizing the raw materials 7, and an access ramp 8 for introducing these raw materials 7 into the tank 15 via the inlet opening 9 of this vessel 15.

The access ramp 8 is a duct whose one end is connected to the outlet of the silo 6 and whose other end opens into the inlet opening 9 of the tank 15.

The raw materials 7 can flow by gravity from the silo 6 to the inlet of the tank 15 via the access ramp 8. A pusher system (not shown) can be provided to force this circulation.

In practice, here said raw materials 7 comprise a raw material source of alumina (Al 2 O 3) and / or aluminum (Al), and a raw material source of calcium oxide (CaO) and / or calcium (Ca) .

Said source material of alumina (Al 2 O 3) and / or aluminum (A 1) denotes any chemical compound comprising an Al 2 O 3 atom group and / or an aluminum atom.

Similarly, said source material of calcium oxide (CaO) and / or calcium (Ca) refers to any chemical compound comprising a GaO atom group and / or a calcium atom.

Thus, alternatively, the silo 6 may optionally comprise two separate compartments (not shown) adapted to respectively receive said source material source of alumina (Al 2 O 3) and / or aluminum (Al), and said source material oxide source calcium (CaO) and / or calcium (Ca). It could for example be envisaged that these separate compartments open downstream in a common part of the silo 6 situated upstream of the outlet of said silo 6. In this common part, said source material of alumina (Al 2 O 3) and / or aluminum (Al) and said source material source of calcium oxide (CaO) and / or calcium (Ca) are then mixed to form the raw materials 7.

Whatever the envisaged variant of the charging system 2, this charging system 2 can feed the melt 11 raw material 7, and this continuously.

In addition, as shown in FIG. 5, the exhaust system 3 of the oven 1 is connected to the outlet opening 12 of the tank 15.

The evacuation system 3 of the liquid calcium aluminate mass 16 comprises a discharge duct 13 connected on one side to the outlet opening 12 of the tank 15, and which opens, on the other side, on a cooling zone (not shown) calcium aluminates. The outlet opening 12 of the tank 15 is an outlet called "overflow" insofar as the melt, namely the molten calcium aluminate, is discharged from the tank 15 by overflow thereof in the duct evacuation 13.

Advantageously, this outflow opening 12 is compatible with the very high temperatures of the melt 11.

Process

In the remainder of the description, we will detail more specifically the manufacturing process of calcium aluminates implemented by an operator in the industrial furnace 1 described above.

Remarkably, according to this method: a) is continuously introduced into the vessel 15 of refractory material containing the melt 11 permanently heated, fine particles of said raw material source of alumina (Al2O3) and / or aluminum (Al) and said source material of calcium oxide (CaO) and / or calcium (Ca) having a median diameter d50 of less than or equal to 6,000 μm for melting said fine particles of raw material, and b) a mass of liquid calcium aluminates 16 is continuously recovered at the outlet of the tank 15.

In a step prior to step a), the operator prepares the melt 11.

For this purpose, when the furnace 1 is put into service, the tank 15 is initially charged with a preliminary mixture of calcium aluminates.

This preliminary mixture is heated by the heating system 10 so as to obtain a liquid mass, without melt, of molten calcium aluminate. This liquid mass then forms the initial melt present in the tank 15 at the beginning of the implementation of the manufacturing method according to the invention.

The melt 11 is formed by this initial melt, to which is added said source material of alumina (Al2O3) and / or aluminum (Al), and said raw material source of calcium oxide (CaO) and / or calcium (Ca), which will melt in turn.

Thus, in the following description, the melt 11 designates a liquid mass, without melt, of molten calcium aluminate.

The volume of the initial melt is such that it is flush with the outflow opening 12 of the vessel 15.

The mass ratio ΑΙ / Ca of the preliminary mixture of calcium aluminate initially loaded in the tank 15, namely the ratio between the total mass of aluminum (Al) and the total mass of calcium (Ca) contained in this preliminary mixture, is close to that of the calcium aluminate that the operator wishes to recover at the outlet of tank 15, but not necessarily identical thereto.

Indeed, the mass ratio ΑΙ / Ca of the calcium aluminate contained in the tank 15 - that is to say forming the melt 11 - evolves during the process, by introducing, at the stage a), raw materials 7. Thus, it should be understood that the ratio / Ca mass ratio of the calcium aluminate recovered at the vessel outlet 15 may be different from that of the calcium aluminate initially loaded into the vessel 15 .

The Al / Ca mass ratio of the calcium aluminate recovered at the vessel outlet 15 tends to become equal to the Al / Ca mass ratio of the raw materials introduced.

Thus, there is a transient regime in which the Al / Ca mass ratio of calcium aluminate recovered at the vessel outlet is different from the Al / Ca mass ratio of the raw materials introduced. At the end of the transient regime, the Al / Ca mass ratio of the calcium aluminate recovered at the vessel outlet becomes equal to the Al / Ca mass ratio of the raw materials introduced at the vessel inlet.

In conventional manner, it is estimated that the duration of the transient regime is at most equal to 5 times the residence time of the particles in the tank 15.

For example, for a residence time of about 1 hour, it is estimated that the transient is completed after 5 hours. In step a), the operator loads the charging system 2 with raw material 7 containing a raw material source of alumina (Al 2 O 3) and / or aluminum (Al), and a raw material source of oxide calcium (CaO) and / or calcium (Ca). At this step a), the operator introduces in the furnace 1, via the inlet opening 9 of the tank 15, in the form of fine solid particles, said source material source of alumina (Al 2 O 3) and / or aluminum (Al), and said raw material source of calcium oxide (CaO) and / or calcium (Ca).

Here, the term "fine particles" means a free powder having a median diameter d50 of less than or equal to 6000 pm.

The loose powder is considered a fractional state of the solid which is then in the form of very small pieces.

Advantageously, a free powder having such a median diameter d50 has a large specific surface favorable for its melting in the melt 11.

Sets of fine particles having a median diameter d50 of less than or equal to 6000 μm are, for example, those having the following median diameters d50: 6 mm; 5 mm; 4 mm; 3 mm; 2 mm; 1 mm; 500 μm; 250 pm; 150 pm; 100 pm; 50 pm; 25 pm, and lower.

Preferably, the median diameter d50 of the fine particles that are suitable for the process according to the invention is greater than or equal to 25 μm and less than or equal to 6 mm.

In fact, fine particles having a median diameter d50 of less than 25 μm could lead to clogging of the furnace 1. Fine particles having a median diameter d50 of greater than 6 mm could in turn reduce the production and / or the quality of the aluminates. calcium by generating unfused in the melt 11, and then out of the tank.

More preferably, the fine particles have a median diameter d50 of between 100 μm and 1000 μm.

Even more preferably, they have a median diameter d50 of between 150 μm and 500 μm.

Ideally, the median diameter d50 of the fine particles is 250 μm.

In addition, the maximum diameter of the fine particles is another characteristic dimension making it possible to choose the fine particles most suitable for carrying out the invention.

The maximum diameter is a reference diameter defined as the diameter below which 100% of the fine particles used are.

In other words, all the fine particles of the set of particles considered have a smaller diameter than the maximum diameter.

Preferably, the fine particles have a maximum diameter of less than or equal to 20,000 μm, that is, less than or equal to 2 cm.

Thus, the sets of fine particles having the following maximum diameters may be suitable for carrying out the invention: 20,000 μm; 19,000 pm; 18,000 pm; 17,000 pm; 16,000 pm; 15,000 pm; 14,000 pm; 13,000 pm; 12,000 pm; 11,000 pm; 10,000 pm; 9,000 pm; 8,000 pm; 7,000 pm; 6000 pm; 5,000 pm; 4000 pm; 3000 pm; 2,000 pm; and lower.

In general, the maximum diameter of the fine particles is chosen so as to ensure the complete melting of all the fine particles during the residence time of these fine particles in the furnace tank. This maximum diameter therefore depends on the size of the oven tank.

The larger the maximum diameter of the fine particles, the longer the residence time for the complete melting of these fine particles increases and the larger the oven size.

Very advantageously, the process according to the invention is easily and rapidly adaptable to many particle sizes.

The maximum diameter of the fine particles used is determined according to the purchase cost and / or production of these fine particles and the size of the oven 1. To date, the particles are all the more expensive as they are small . It is therefore economically interesting to use the largest particles possible. But a long tank length imposes a large oven size, so a higher cost for the construction and maintenance of this oven.

In the event that the cost of purchasing and / or producing the smaller particles decreases, it would probably be advantageous to use these particles rather than coarse particles, and the maximum particle diameter could be lowered. By way of example, FIG. 1 shows the granulometric curve of a first set of fine particles of bauxite that can be used in the process according to the invention.

In this FIG. 1, the ordinate axis gives the quantity of particles expressed as a percentage by weight relative to the total mass of the total quantity of particles, and the abscissa axis gives the diameter of the particles in micrometers (μm) on a scale. logarithmic.

The particle size curve represented here is a so-called "cumulative" curve, that is to say that each point of this granulometric curve represents the percentage of particles having a diameter less than or equal to that corresponding to the point of the curve studied.

For example, this granulometric curve indicates that 70% of the particles of the first set of particles have a diameter less than or equal to 100 μm.

Similarly, in this first set of particles, the median diameter d50 is equal to 60 μm, that is to say that 50% of the particles of the first set have a diameter of less than or equal to 60 μm.

The maximum diameter of the particles here is 300 μm, which means that 100% of the particles of this first set of particles have a diameter less than or equal to 300 μm.

FIGS. 2 to 4 show the particle size diagrams of the second, third and fourth sets of fine particles that can be used in the process according to the invention.

The bar graphs of FIGS. 2, 3 and 4 give the mass percentage of particles having a diameter less than or equal to that indicated at the bottom of each bar.

For example, the bar chart of Figure 2 indicates that the median diameter d50 of the second set of particles suitable for the invention is between 0.5 millimeters (mm) and 1 mm. It can be estimated that said median diameter d50 in this case is about 0.9 mm.

In this Figure 2, we see that the maximum particle diameter of this second set is 2 mm.

Similarly, the bar graphs of FIGS. 3 and 4 respectively indicate that the median diameter d 50 of the third set of fine particles suitable for the invention is between 1 mm and 2 mm, and that the median diameter d 50 of the fourth set of fine particles suitable for the invention is between 2 mm and 3.15 mm.

It can be estimated that said median diameter d50 is about 1.4 mm for the third set of particles corresponding to the diagram shown in Figure 3, and about 3 mm for the fourth set of particles corresponding to the diagram shown in Figure 4.

The maximum particle diameter of the third set corresponding to the diagram shown in FIG. 3 is 4 mm. It is 20 mm for the fourth set of particles corresponding to the diagram shown in FIG. 4.

In general, the median diameter d50 of the fine particles may vary according to the type of raw materials used during the implementation of the process according to the invention.

In particular, the median diameter d50 of the fine source material particles of alumina and / or aluminum may be different from that of the fine source particles of calcium oxide and / or calcium.

Advantageously, it will be possible according to the invention to size the source materials of alumina (Al2O3) and / or aluminum (Al) and source of calcium oxide (CaO) and / or calcium (Ca ) using a grinder before introducing them into the melt 11.

In other words, it will be possible to reduce the diameter of the raw material particles so as to obtain a set of fine particles whose median diameter d50 is as desired.

In addition, preferably, the source material of alumina (Al 2 O 3) and / or aluminum (Al) comprises, by weight relative to the total mass of said source material of alumina (Al 2 O 3) and / or of aluminum (Al), at least 30%, or even at least 40% or 50%, of alumina (Al2O3) and / or aluminum (Al).

Preferably, the source material of calcium oxide (CaO) and / or calcium (Ca) comprises, by weight relative to the total mass of said source material source of calcium oxide (CaO) and / or calcium (Ca), at least 50%, or even at least 70% or 90%, of calcium oxide (CaO) and / or calcium (Ca).

In the manufacturing method according to the invention, the source material source of alumina (Al 2 O 3) and / or aluminum (Al) is preferably chosen from: bauxite such as bauxite monohydrate and / or bauxite trihydrate, white bauxite, red bauxite, corundum wheels, catalyst supports, refractory bricks, hydroxides, metallurgical aluminas, calcined and melted aluminas, by-products of the aluminum die and non-compliant high alumina content or a mixture thereof.

Preferably, during the implementation of the method according to the invention in the variant of the furnace having the electric heating means, the source material source of alumina (Al 2 O 3) and / or aluminum (Al) contains little iron ( Fe).

Also preferably, the source material source of calcium oxide (CaO) and / or calcium (Ca) is chosen from: limestone, lime and by-products from consumer processes of limestone and / or lime as slag or slag from iron and steel or electrometallurgy, or a mixture thereof.

In particular, the raw materials source of alumina (Al2O3) and / or aluminum (Al) and source of calcium oxide (CaO) and / or calcium (Ca) used in step a) can present the compositions described in the following Tables 3 and 4:

Table 3

Table 4

The raw materials source of alumina (A203) and / or aluminum (Al), and source of calcium oxide (CaO) and / or calcium (Ca) may also contain iron (Fe) and silica (Si02) in variable quantity. For example, bauxite trihydrate may comprise by weight, from 46% to 50% of alumina (Al 2 O 3), from 14% to 20% of iron oxide under varying degrees of oxidation and from 7% to 12% of silica. (Si02).

In addition, the raw materials source alumina (Al203) and / or aluminum (Al), and source of calcium oxide (CaO) and / or calcium (Ca) introduced in step a) of the process according to the invention are preferably dosed so that the mass ratio ΑΙ / Ca in the finished product, that is to say in the calcium aluminate recovered at the vessel outlet 15, is between 0.5 and 1, 7, and preferably between 0.9 and 1.5.

Even more preferably, the weight ratio of aluminum (Al) to calcium (Ca) in the calcium aluminate recovered at the vessel outlet 15 is between 1 and 1.1.

In order to respect this mass ratio, the source material of alumina (Al 2 O 3) and / or aluminum (Al) and the source material of calcium oxide (CaO) and / or calcium (Ca) are metered, either when they are in the form of fine particles having the desired median diameter d50, or before they are reduced to present said desired median diameter d50.

Then, they are mixed in a mixer to form the raw materials 7 which are introduced into the tank 15 of the oven 1. This mixture is performed on the raw materials reduced to the state of fine particles of median diameter d50 desired. The operator can perform this assay and mixing before said raw materials source of alumina (Al 2 O 3) and / or aluminum (Al) and source of calcium oxide (CaO) and / or calcium (Ca) are transported to the charging system 2 of the oven 1 and in particular to the storage silo 6. This transport can be carried out by means of a pump or any other means of transfer.

Alternatively, the metering and blending of said raw material source of alumina (Al2O3) and / or aluminum (Al) and source of calcium oxide (CaO) and / or calcium (Ca) can be carried out directly. in the silo 6 of the charging system 2 of the furnace 1 when it is provided with two separate compartments opening into the common part of the silo 6.

Advantageously, according to the manufacturing method according to the invention, in step a), the temperature of the melt 11 is between 1300 ° C. and 1700 ° C.

This temperature is preferably between 1400.degree. C. and 1600.degree.

In addition, particularly advantageously, the present invention proposes to control the partial pressures of gas contained in the enclosure formed by the vault 5 and the tank 15 of the furnace 1, so as to obtain calcium aluminates having a controlled mineralogy.

Thus, advantageously, also in step a), the melt 11 is placed under a reducing atmosphere.

In chemistry, very generally, a reducing agent is a chemical species capable of yielding one or more electrons to another chemical species, called an oxidizer, during a redox reaction. Conversely, an oxidant is a chemical species capable of capturing one or more electrons during a redox reaction.

The term "reducing atmosphere" here means an atmosphere whose oxidation capacity has been reduced by decreasing the proportion of oxidant that it contains.

In particular, here the reducing atmosphere contains a reduced content, relative to air, of O 2 O 2, which is an oxidizing compound.

In addition, here, the reducing atmosphere comprises a proportion of gases which are more reducing than air in general, and that oxygen in particular.

Here, said reducing atmosphere under which is placed the melt 11 comprises carbon monoxide (CO).

In particular, the method according to the invention proposes to control the content of carbon monoxide (CO) contained in the atmosphere above the melt 11.

Said average carbon monoxide (CO) content of the reducing atmosphere is here between about 0.1% and 100%.

The content of the carbon monoxide (CO) reducing atmosphere may for example be equal to 0.09%; 0.1%; 0.15%; 0.2%; 0.5%; 0.8%; 1%; 5%; 10%; 20%; 30% ; 40%; 50%; 60%; 70%; 80%; 90%; or 100%, in mol relative to the amount of total material of gas analyzed.

In a particularly advantageous manner, the control of the carbon monoxide (CO) content of the reducing atmosphere situated above the melt 11 makes it possible to control the mineralogical phases, and in particular the C12A7 mineralogical phase, of the calcium aluminates obtained. at the outlet of the tank 15.

Indeed, the applicant has discovered that when the atmosphere above the melt 11 is rich in carbon monoxide (CO), the calcium aluminates obtained are rich in C12A7 phase. The opposite is also true.

However, it is particularly advantageous to control the proportion of C12A7 mineralogical phase contained in the calcium aluminate recovered at the tank outlet 15 since it turns out that this C12A7 mineralogical phase influences the reactivity of this calcium aluminate used as a binder. hydraulic.

More specifically, the C12A7 mineralogical phase is an accelerator for setting calcium aluminates. In other words, the calcium aluminates harden all the faster in contact with water they contain a large proportion of C12A7 mineralogical phase, compared to other mineralogical phases optionally contained in these calcium aluminates.

Thus, the process according to the invention advantageously makes it possible to control the rate of setting of the calcium aluminates produced.

In addition, advantageously, according to the invention, in step a), the residence time of said fine particles in said melt 11 is less than 24 hours.

This residence time is preferably between 30 minutes and 9 hours and even more preferably, it is equal to 8 hours.

Particles not all having exactly the same diameter do not all move at the same speed in the tank 15. Therefore, the residence time is here an average residence time of said fine particles in the melt 11. After this residence time, the operator obtains at the tank outlet 15 the desired product, namely the mass of liquid calcium aluminates 16, without being unfilled with raw material.

The mass of liquid calcium aluminates 16 is the result of a set of progressive physicochemical reactions allowing the drying, dehydration and decarbonation of the source materials of alumina (Al 2 O 3) and / or aluminum (Al). and source of calcium oxide (CaO) and / or calcium (Ca) to form calcium aluminate.

During the process according to the invention, these physico-chemical reactions are carried out in the melt 11 contained in the tank 15 of the oven 1 and this, in a single step, which is an advantage as regards the operating costs .

During these physicochemical reactions, and during the melting of the raw materials 7, bubbles are generated in the melt 11.

These bubbles participate by stirring in the homogenization of the melt 11, namely the molten calcium aluminate bath.

Alternatively, it can be provided that the drying, dehydration and decarbonation of the fine particles are initiated during a preheating step prior to step a). According to this variant, no bubble is generated in the melt 11 to the extent that there is no decarbonation there. In step b), after their stay in the melt 11, the fine particles of raw material having become a mass of liquid calcium aluminates, the volume of the melt 11 increases so that the mass of Liquid calcium aluminate 16 overflows through outlet opening 12 of vessel 15.

This mass of liquid calcium aluminate 16 is discharged through the exhaust duct 13 of the exhaust system 3 of the furnace 1.

Thus, during the operation of the furnace 1, the introduction of the fine solid particles of raw material into the tank 15 increases the volume of the melt 11, and a portion of the mass of liquid calcium aluminates 16 present in said tank 15 ends up overflowing through the outlet opening 12 of the tank 15.

Steps a) and b) are both repeated continuously and simultaneously.

Thus, simultaneously, at the inlet opening 9 of the tank 15, the fine solid particles are introduced into the tank 15 (step a), and at the outlet opening 12, the aluminate mass liquid calcium 16 is recovered (step b), without stopping.

The fine particles circulate slowly, due to the viscosity of the melt 11, between the inlet opening 9 and the outlet opening 12. Thus they stay in the tank 15 long enough to melt and react in order to increase the mass of liquid calcium aluminate.

The flow of material in the tank 15, from the inlet opening 9 to the outlet opening 12, is generated by the evacuation of the calcium aluminate mass 16 via the outlet opening 12.

Thus, remarkably, the process according to the invention makes it possible to manufacture calcium aluminates continuously.

In a step subsequent to step b), the operator chills the mass of liquid calcium aluminates.

Preferably, this cooling occurs naturally, that is to say at room temperature in the cooling zone connected to the evacuation system 3.

Once the cooling is complete, the mass of calcium aluminates 16 is in the form of clinker. Clinker is a hardened mass of calcium aluminate. Depending on the type of cooling chosen, the clinker can take different shapes and sizes. In general, this clinker is in the form of hard granules which have a diameter of up to a few tens of centimeters.

The clinker thus obtained is then preferably evacuated by carpet or any other conveying means to a hall, or a silo, where it will eventually be stored.

This clinker may optionally be milled, more or less finely, to form a calcium aluminate cement, namely a calcium aluminate in the form of powder which has active hydraulic properties.

Preferably, the grinding of said clinker is carried out by means of a ball mill (cylindrical device rotated lined with shielding plates and loaded with steel balls).

Alternatively, grinding can be carried out in vertical grinding mills, or in any type of mill having sufficient wear resistance and suitable for small diameter reduction.

The calcium aluminate cements thus obtained can be used in applications such as building chemistry or refractory chemistry.

In addition, the calcium aluminate cements can then undergo different treatments or be mixed with other compounds. For example, the addition of additives such as calcium sulphate, fly ash, pozzolans, gypsum is possible in order to valorize calcium aluminate cement in various applications.

Examples

The following Examples 1 to 4 make it possible to account for the importance of the carbon monoxide (CO) content in the atmosphere of the enclosure for controlling the mineralogical phases of the calcium aluminate cements obtained.

In addition, these examples prove that the process according to the invention makes it possible to obtain calcium aluminates comprising a controlled content of C12A7 mineralogical phase.

Below, Table 5 shows all the experimental conditions used to carry out Examples 1 to 4.

In particular, in this table 5, the content of carbon monoxide (CO) is given as a percentage by weight, that is to say by weight of carbon monoxide (CO) relative to the total mass of the constituents of the atmosphere.

In addition, the mass ratio Al / Ca corresponds to the total weight of aluminum reported on the total mass of calcium, contained in the final calcium aluminate obtained. Thus, the amounts of aluminous and calcareous raw materials introduced into the furnace are calculated so as to respect this mass ratio Al / Ca in the final product. Before their introduction in the furnaces, these raw materials are mixed and homogenized.

The diameter d50 corresponds to the median diameter, given in micrometers, of all the fine particles of raw materials introduced into the furnaces.

Finally, the residence time corresponds to the average residence time of the fine particles in the melt. It is given in hour.

In Example 1, the oven used is a combustion oven. The carbon monoxide (CO) content is inherent to combustion. It is measured at the level of the smoke exhaust opening of the oven. The raw materials used are so-called white bauxites, containing very little or no iron, that is to say less than 10%, preferably less than 5%, of iron, and limestone.

In Examples 2, 3 and 4, the oven used is an electric furnace equipped with a gas addition system. The raw materials used are also white bauxite and limestone.

In Example 2, the content of carbon monoxide (CO) is chosen to be 100%. This carbon monoxide (CO) is directly introduced into the furnace by the gas addition system.

In Examples 3 and 4, the carbon monoxide content is chosen to be 0%.

In Example 3, the gas addition system introduces dinitrogen (N2), so that the melt is placed in a 100% nitrogen atmosphere.

In Example 4, the gas addition system introduces air at atmospheric pressure, so that the melt is placed in the air.

Table 5

The final composition of the calcium aluminate cements obtained is evaluated according to an X-ray diffraction method.

This X-ray diffraction method is described in more detail in Calcium Aluminate Cernent: Proceeding of the Centenary Conference Avignon 30 June 2008 "Quantitative Mineralogical Chemical and Application Investigations of High Alumina Cements from Different Sources" H. Pollmann & Al.

In particular, the X-ray diffraction method employed meets the respective French and European standards in force NF EN 13925-1 and EN 13925-1.

The composition of the calcium aluminate cement obtained in Example 1, respectively in Examples 2 to 4, is given in Table 6, respectively in Table 7, below, in mass relative to the total mass of the composition of the cements:

Table 6

Table 7

Moreover, in other examples of implementation of the process according to the invention, calcium aluminates were obtained in the oven of Example 1, from the same raw materials as those used in Example 1, but having respectively a median diameter d50 of the order of 400 pm and 2 mm.

These calcium aluminates did not have an unfused furnace outlet, despite the greater median diameter d50 of the raw materials introduced into the furnace.

In a last example of implementation of the process according to the invention, a calcium aluminate was also obtained in the oven of Example 1, from red bauxites and limestone, all the other experimental conditions being identical to those described in Example 1.

All the calcium aluminates obtained in the examples described above are intended for applications in building chemistry, or in the field of refractory concretes.

Claims (14)

A process for the production of calcium aluminates in an industrial furnace (1), in which: a) is introduced continuously into a vessel (15) of refractory material containing a permanently heated melt (11), fine particles of a source material of alumina (Al 2 O 3) and / or aluminum (Al) and a source material of calcium oxide (CaO) and / or calcium (Ca) having a diameter median d50 less than or equal to 6,000 pm for melting said fine particles of raw material, and b) is recovered continuously at the tank outlet (15) a mass of calcium aluminates (16) liquid. 2. The manufacturing method according to claim 1, wherein in step a), the melt (11) is placed under a reducing atmosphere comprising carbon monoxide (CO). 3. The manufacturing method according to one of claims 1 and 2, wherein in step a), said reducing atmosphere comprises on average from 0.1% to 100% carbon monoxide (CO). 4. The manufacturing method according to one of claims 1 to 3, wherein the temperature of the melt (11) of calcium aluminates is between 1300 ° C and 1700 ° C. 5. The manufacturing method according to claim 4, wherein the temperature of the melt (11) of calcium aluminates is between 1400 ° C and 1600 ° C. 6. The manufacturing method according to one of claims 1 to 5, wherein the residence time of said fine particles of raw material in said melt (11) of calcium aluminates is less than 24 hours. 7. The manufacturing method according to claim 6, wherein the residence time of said fine particles of raw material in said melt (11) calcium aluminates is between 30 minutes and 9 hours. 8. The manufacturing method according to one of claims 1 to 7, wherein in step a), the source material source of alumina (Al 2 O 3) and / or aluminum (Al) introduced into the vessel (15). ) is selected from: bauxite, corundum wheels, catalyst supports, refractory bricks, hydroxides, metallurgical aluminas, calcined and melted aluminas, by-products of the aluminum die and non-conforming high content of alumina or a mixture thereof, and the raw material source of calcium oxide (CaO) and / or calcium (Ca) introduced into the tank (15) is chosen from: limestone, lime and subsoil -products derived from consumer processes of limestone and lime, such as slag or slag from iron and steel or electrometallurgy, or a mixture thereof. 9. The manufacturing method according to one of claims 1 to 8, wherein the fine particles of raw material have a median diameter d50 between 100 pm and 1000 pm. 10. The manufacturing method according to claim 9, wherein the fine particles of raw material have a median diameter d50 between 150 pm and 500 pm. 11. The manufacturing method according to one of claims 1 to 10, wherein after step b), the mass of liquid calcium aluminates (16) recovered at the tank outlet (15) is cooled. 12. The manufacturing method according to claim 11, wherein the cooling is carried out naturally. 13. The manufacturing method according to one of claims 11 and 12, wherein the mass of calcium aluminates (16) is cooled cooled to form a calcium aluminate cement. 14. The manufacturing method according to one of the preceding claims, wherein said fine particles of raw material source of alumina (Al2O3) and / or aluminum (Al) and raw material source of calcium oxide (CaO) and / or calcium (Ca) are introduced into the vessel (15) in the form of a free powder.